Built Environment, Urban Systems, and Cities

Figure 11.1: Current and Projected U.S. Population

Figure 11.1: These maps show current population along with population projections by county for the year 2100. Projected populations are based on Shared Socioeconomic Pathways (SSPs)—a collection of plausible future pathways of socioeconomic development.8 The middle map is based on demography consistent with the SSP2, which follows a middle-of-the-road path where trends do not shift markedly from historical patterns. The bottom map uses demography consistent with SSP5, which follows a more rapid technical progress and resource-intensive development path. Increasing urban populations pose challenges to planners and city managers as they seek to maintain and improve urban environments. Data are unavailable for the U.S. Caribbean, Alaska, and Hawai‘i & U.S.-Affiliated Pacific Islands regions. Source: EPA

Urban areas in the United States, where the vast majority of Americans live, are engines of economic growth and contain land valued at trillions of dollars. In 2015, nearly 275 million people (about 85% of the total U.S. population) lived in metropolitan areas, and 27 million (about 8%) lived in smaller micropolitan areas.4 Metropolitan areas accounted for approximately 91% of U.S. gross domestic product (GDP) in 2015, with over 23% coming from the five largest cities alone.5 Urban land values are estimated at more than two times the 2006 national GDP.6 Urbanization trends are expected to continue (Figure 11.1), and projections suggest that between 425 and 696 million people will live in metropolitan and micropolitan areas combined by 2100.7 All of these factors affect how urban areas respond to climate change.

Cities around the United States face a number of challenges to prosperity, such as social inequality, aging and deteriorating infrastructure, and stressed ecosystems. Urban social inequality is evident in disparities in per capita income, exposure to violence and environmental hazards, and access to food, services, transportation, outdoor space, and walkable neighborhoods.9,10,11,12 Cities are connected by networks of infrastructure, much of which is in need of repair or replacement. Failing to address aging and deteriorating infrastructure is expected to cost the U.S. GDP as much as $3.9 trillion (in 2015 dollars) by 2025.13 Current infrastructure and building design standards do not take future climate trends into account.14 Urbanization affects air, water, and soil quality and increases impervious surface cover (such as cement and asphalt).15,16,17 Urban forests, open space, and waterways provide multiple benefits, but many are under stress because of land-use change, invasive species, and pollution.18 These social, infrastructure, and environmental challenges affect urban exposure and susceptibility to climate change effects.

Urban areas, where the majority of the U.S. population lives and most consumption occurs, are the source of approximately 80% of North American human-caused greenhouse gas (GHG) emissions, despite only occupying 1%–5% of the land. Therefore, changes to urban activities can have a significant impact on national GHG emissions.19 Land use and land-cover change contribute to radiative forcing, and infrastructure design can lock in fossil fuel dependency, so urban development patterns will continue to affect carbon sources and sinks in the future (Ch. 5: Land Changes).19,20,21 Many cities in the United States are working to reduce their GHG emissions and can be key leverage points in mitigation efforts.